Varied and Inhospitable

To the Editors:

Kudos to American Scientist for Howard A. Smith’s “Alone in the Universe,” (July–August) which contradicts the popular idea of a universe full of intelligent life. But Smith largely neglects this idea’s history—a history which suggests an important lesson to be learned about our presumptions.

After Copernicus proposed that Earth was one of several planets circling the Sun, astronomers (including Kepler, Huygens and Herschel) presumed that the planets were similar to Earth in various ways, including the presence of intelligent life. In an introduction to the 1803 edition of Bernard Fontenelle’s 17th-century classic, Conversations on the Plurality of Worlds, astronomer Jérôme LaLande wrote that

there is every possible resemblance between the planets and the earth: Is it, then, rational to suppose the existence of living and thinking beings is confined to the earth? From what is such a privilege derived but the groveling minds of persons who can never rise above the objects of their immediate sensations?

The idea of an inhabited solar system (within an inhabited universe) was promoted into the late 19th century.

But were LaLande alive today, he would see that the planets in the solar system are amazingly diverse, and only Earth harbors thinking beings. As Smith points out, extrasolar planets are likely to be varied as well. History reveals that we presume too much resemblance and too little diversity among other worlds and Earth. We are already far more alone than centuries of astronomers would ever have guessed.

Christopher M. GraneyLouisville, KY

To the Editors:

Like Howard Smith, I am skeptical about life on other planets. In my experience, astronomers tend to be far more optimistic about the possibilities than do biologists. In fact, I suspect that the necessary conditions for life may be even more stringent than those Smith presents in his article. He discusses the habitable zone, the region around a star in which temperatures would allow a planet to have liquid water. The notion of a habitable zone was discussed in 1964 in a thin book entitled Habitable Planets for Man, by Stephen Dole. That was before x-ray and ultraviolet telescopes revealed that the most common stars in the galaxy, those less massive and cooler than the Sun (the red dwarfs), are powerful x-ray and ultraviolet sources. Planets in the habitable zones of these stars may be water worlds, but they will not be awfully hospitable because of this radiation. Planet hunters have begun to search for exoplanets by direct imaging, and red stars offer the most favorable light ratio for this search. The discoveries in these stars’ habitable zones will be called habitable planets, but are they?

Michal SimonStony Brook University

Howard Smith responds:

I thank Christopher Graney for pointing out the delicious comments of the noted astronomer Jérôme LaLande. His view, which is still common, presumes ordinariness for Earth and humanity—based largely on the rejection of religious imperatives about our specialness, rather than on evidence. Such unproven faith in our quotidian nature is no more warranted than is the opposite presumption. I based my article on a talk for a panel held by the American Association for the Advancement of Science on science, religion, ethics and the implications of discovering extraterrestrial life. Today, a frequent assumption among atheists and theists alike is that aliens are everywhere. The world’s major religions have accepted the idea as a manifestation of God’s universal creative power. Moreover, many people hope alien civilizations can help us solve our problems—God forbid, some might say, that we ourselves should be cosmically relevant, significant or, worse, responsible. This attitude is why I think religion and ethics would do better asking about the implications of not discovering extraterrestrial life! I hope that my talk and this paper will promote conversations about our responsibilities in the likely situation that the Earth is precious and life is rare; about how so-called esoteric, basic science can dramatically reshape our self image and our culture; and about the importance of recognizing the commonalities of science and religion.

My talk was long but perforce this article was short. I thank my colleague Michal Simon for the opportunity to echo his observation about low-mass stars, which are the subject of important new programs, such as the MEarth Project, that search for habitable worlds around abundant “M dwarf” stars. Unfortunately these stars are not only likely to have strong x-ray emissions, they are also highly variable in luminosity and subject to flaring. These conditions do not necessarily rule out the possibility of life, but they make it harder for life to originate and evolve.